The present invention relates to an RF power amplifier and an operating method thereof, and particularly relates to a technology effective for reducing the size of a power detection circuit.
Cell phones, which provide mobile communications, have progressed from the second generation (2G) represented by the GSM standard to the third generation (3G) represented by the W-CDMA and CDMA2000 standards. The cell phones have further progressed to the LTE standard, which is intermediate between the third generation (3G) and the forthcoming fourth generation (4G). GSM is an acronym for Global System for Mobile Communications. W-CDMA is an acronym for Wideband Code Multiplex Access. LTE is an acronym for Long Term Evolution.
The GSM standard, which is the second generation (2G), includes four standards: GSM850, GSM900, DCS1800, and PCS1900. The GSM850, which is widespread mainly in North America, uses radio frequencies (RF) between 824 MHz and 915 MHz for the transmission (uplink) from a cell phone terminal to a base station. The GSM900, which is widespread mainly in Asia, Africa, Oceania, Europe, and Latin America, uses radio frequencies (RF) between 880 MHz and 915 MHz for the transmission (uplink) from a cell phone terminal to a base station. The DCS1800, which is also widespread mainly in Asia, Africa, Oceania, Europe, and Latin America, uses radio frequencies (RF) between 1710 MHz and 1785 MHz for the transmission (uplink) from a cell phone terminal to a base station. The PCS1900, which is widespread mainly in North America and Latin America, uses radio frequencies (RF) between 1850 MHz and 1910 MHz for the transmission (uplink) from a cell phone terminal to a base station. The GSM850 and GSM900 standards are generally called low-band GSM standards, whereas the DCS1800 and PCS1900 standards are generally called high-band GSM standards.
According to technical specifications for the LTE standard, which are prepared by the 3GPP, an extremely large number of frequency bands are available. Typically, band 1, band 2, band 4, band 5, and band 8 are available. For the transmission (uplink) from a band 1 cell phone terminal to a base station, radio frequencies (RF) between 1920 MHz and 1980 MHz are used. For the transmission (uplink) from a band 2 cell phone terminal to a base station, radio frequencies (RF) between 1850 MHz and 1910 MHz are used. For the transmission (uplink) from a band 4 cell phone terminal to a base station, radio frequencies (RF) between 1710 MHz and 1785 MHz are used. For the transmission (uplink) from a band 5 cell phone terminal to a base station, radio frequencies (RF) between 824 MHz and 915 MHz are used. For the transmission (uplink) from a band 8 cell phone terminal to a base station, radio frequencies (RF) between 880 MHz and 915 MHz are used. 3GPP is an acronym for Third Generation Partnership Project.
Meanwhile, using a directional coupler for providing power control of an RF power amplifier is described in “Accurate Power Control Technique for Handset PA Modules with Integrated Directional Couplers” (Jelena Madic et al., 2003 IEEE Radio Frequency Integrated Circuits Symposium, pp. 715-718). According to this document, the power generated from the RF power amplifier is supplied to a main line for a directional coupler. One end of an auxiliary line for the directional coupler is coupled to a ground potential through a terminating resistor. A voltage detected at the other end of the auxiliary line for the directional coupler and a control voltage are supplied to a controller. The RF power amplifier is controlled in accordance with an output from the controller. The detected voltage is determined by adding the vector of a coupled voltage of a traveling-wave signal, which is generated from the RF power amplifier, to the vector of a coupled voltage of a reflected-wave signal, which is reflected from a load.
Further, exercising power control called AC sensing is described in “A VSWR-Protected Silicon Bipolar RF Power Amplifier with Soft-Slope Power Control” (Angelo Scuderi et al., IEEE Journal of Solid-State Circuits, Vol. 40, No. 3, March 2005, pp. 611-621) in addition to using a directional coupler for providing power control of an RF power amplifier, as is the case with “Accurate Power Control Technique for Handset PA Modules with Integrated Directional Couplers” (Jelena Madic et al., 2003 IEEE Radio Frequency Integrated Circuits Symposium, pp. 715-718). In AC sensing, the collector voltage of an output transistor at the final stage of the RF power amplifier is supplied to an input terminal of an envelope detector through a coupling capacitor. An error amplifier compares a voltage detected by the envelope detector to a reference voltage. A driving stage for driving the output transistor at the final stage is controlled in accordance with an output from the error amplifier.
Power control called current sensing is described in U.S. Pat. No. 6,307,364 B1 in addition to power control exercised while a directional coupler formed by a pair of transmission lines is coupled to the collector of an RF amplifying transistor of an RF power amplifier through an output matching circuit. For purposes of current sensing, the base of a small RF signal sensing transistor is coupled to the base of the RF amplifying transistor through two bias resistors. A common bias voltage is supplied to these transistor bases through the two bias resistors. Further, a common RF input signal is supplied through two coupling capacitors. As the element size of the RF signal sensing transistor is set to be physically smaller than that of the RF amplifying transistor, the RF amplification signal of the RF signal sensing transistor is smaller than the RF amplification signal of the RF amplifying transistor.